1. Field of the Invention
This invention relates to a brake system for a vehicle which satisfies requirements such as safety, sophisticated control, improved pedal feeling and reduction in cost.
2. State of the Prior Art
If a large effective sectional area of a static pressure generator, i.e. a master cylinder, is adopted to reduce the pedal stroke, and a consequent increase in the pedal force is compensated for by an appropriate booster, the pedal force in the failure of a dynamic pressure source will become extremely large.
On the other hand, the reduction of the sectional area in an attempt to lighten the pedal force will bring about an increase in the pedal stroke. In both cases, the pedal feel is unsatisfactory.
The present applicant proposed in Japanese Patent Application 1-85176 a brake system which can attain both the shortening of the pedal stroke in normal conditions and the lightening of the pedal force in case of the failure of the dynamic pressure line, and furthermore enables the relation between the pedal stroke and the deceleration to be set in any desired manner. In this brake system, pressure fluid from the dynamic pressure source is introduced to the static pressure line and the pressure fluid in the static pressure line is discharged into a reservoir by executing electronic control so that the pedal stroke and the vehicle deceleration will be in a predetermined relationship with each other.
The prior art system was unsatisfactory in that:
1) If even a very small amount of pressure fluid should leak through a flow control valve while the supply of pressure fluid from the dynamic pressure source to the static pressure line ceases due to a failure in the dynamic pressure line or the electronic control, the pressure fluid in the static pressure line might leak out gradually while braking. In order to assure safety, it is necessary to provide some means for preventing such a leakage.
2) If the static pressure line shows a partial failure, that is, if a medium degree of leakage takes place, the pressure fluid of the dynamic pressure source might be consumed to a considerable degree before the leakage is detected.
3) If a flow-variable control valve is used to upgrade antilock control, it is necessary to provide an extra flow-variable control valve for the stroke control. This will increase the cost.